Anatomy of the middle-turn cochleostomy.
OBJECTIVE
Middle-turn cochleostomies are occasionally used for cochlear implant electrode placement in patients with labyrinthitis ossificans. This study evaluates the anatomic characteristics of the middle-turn cochleostomy and its suitability for placement of implant electrodes.


METHODS
Ten cadaveric human temporal bones were dissected using a facial recess approach. A middle-turn cochleostomy was drilled 2 mm anterior to the oval window and just inferior to the cochleariform process. The preparations were then stained with osmium tetroxide and microdissections were performed. The location of the cochleostomy on the cochlear spiral and its path through the various cochlear compartments were evaluated in all 10 specimens. A Cochlear Corporation depth gauge was inserted in five of the specimens and insertion trauma, number of contact rings, and depth of insertion were recorded.


RESULTS
Eight of the 10 cochleostomies were placed at approximately 360 degrees on the cochlear spiral, near the transition between the basal and middle turns. In one case, the cochleostomy was found to enter the cochlear apex and in another it entered scala vestibuli of the proximal basal turn. The cochleostomy entered scala media in six bones and scala vestibuli in four specimens. A depth gauge was inserted in five specimens. The number of contacts placed within the cochlear lumen ranged from four to nine. There was evidence of insertional trauma to the lateral wall of the cochlear duct, basilar membrane, and Reissner's membrane, but no evidence of fractures to the osseous spiral lamina or modiolus.


CONCLUSIONS
This study demonstrates that electrodes inserted via a middle-turn cochleostomy are likely to enter scala vestibuli and have access to the middle- and apical-cochlear turns. It is also possible that the electrode could be directed into the descending portion of the basal turn depending on cochleostomy orientation. Middle-turn cochleostomy seems to be a viable alternative for electrode placement when preservation of residual hearing is not a concern.
Isaacson
Brandon
B
Department of Otolaryngology, Head and Neck Surgery, Southwestern Medical Center, Dallas, TX 75390-9035, USA. brandon.isaacson@utsouthwestern.edu
Roland
Peter S
PS
Wright
Charles G
CG
D001489:Basilar Membrane / Q000473:pathology / Q000601:surgery; D003051:Cochlea / Q000473:pathology* / Q000601:surgery*; D019929:Cochlear Implantation; D004567:Electrodes, Implanted; D006801:Humans; D007762:Labyrinthitis / Q000473:pathology / Q000601:surgery; D009999:Ossification, Heterotopic / Q000473:pathology / Q000601:surgery; D010046:Oval Window, Ear / Q000473:pathology / Q000601:surgery; D012533:Scala Tympani / Q000473:pathology / Q000601:surgery; D055493:Spiral Ligament of Cochlea / Q000473:pathology / Q000601:surgery; D013701:Temporal Bone / Q000473:pathology / Q000601:surgery